A way of final disposal of radioactive waste is retrievable storage or ground disposal wherein the radioactive waste has to be treated to give a solidified waste constructed of a solidified body for this purpose. Cement has been used as the solidifying filler in forming the solidified radioactive waste, but lately there was developed an alkali silicate (solution), as a replacement of cement, which is most suited for the solidification and disposal of the pelletized radioactive waste with high volume, reduction (see Japanese laying-open patent publication No. 57-197,500(1982) published on Dec. 3, 1982).
It has been confirmed that when radioactive waste is treated into a solid body by using a solidifying agent prepared by mixing the alkali silicate (solution) as the solidifying filler, an inorganic phosphate compound (P.sub.2 O.sub.5 :SiO.sub.2) as hardening agent and cement as a water absorbent, the formed solid body has favorable properties, such as high strength, high heat resistance, good durability, and the like. It was found, however, that easily soluble salts are deposited on the surface of the solidified body after hardened. In the course of the solidifying process, the solidifying agent undergoes reactions represented by the following formulae (1) and (2): EQU M.sub.2 O.nSiO.sub.2.xH.sub.2 O+P.sub.2 O.sub.5.SiO.sub.2 nSiO.sub.2 +M.sub.3 PO.sub.4 +xH.sub.2 O (1) EQU CaO.SiO.sub.2 +xH.sub.2 O CaO.SiO.sub.2.xH.sub.2 O (2)
wherein M represents an alkali metal.
The above formulae (1) and (2) correspond to the hardening of the alkali silicate solution by the inorganic phosphate compound and the absorption of the generated water by cement, respectively. The salt M.sub.3 PO.sub.4 (actually a mixture of M.sub.2 HPO.sub.4, MH.sub.2 PO.sub.4, M.sub.2 H.sub.2 P.sub.2 O.sub.7, M.sub.3 PO.sub.4 and their hydrates) produced in the hardening reaction (1) is an easily soluble matter, with its solubility being about 30 wt%, and is dissolved in the liberated water generated in the course of the hardening reaction (1). This dissolving reaction is competitive with the water absorbing reaction, but the former reaction advances faster than the latter since both the salt and the liberated water are formed in the same reaction. The undissolved portion of the salt remains in the hardened solidified body, but the dissolved salt migrates within the solidified body. Thus, as the phenomena observed in the hardened solidified body, there takes place the migration of the liberated water (solution) in which said salt has been dissolved and the evaporation of water from the solidified body surface. Consequently, the liberated water is shifted toward the solidified body surface due to the capillary action and is evaporated therefrom, leaving the recrystallized salt on the solid body surface. This accounts for the phenomenon of salt deposition.
The deposited salt, which is easily soluble as mentioned before, deteriorates the water resistance of the solidified radioactive waste, inviting the danger of causing leakage of radioactive nuclides into the environment. The salt deposition caused when the solidified body made by using an inorganic phosphate compound as hardening agent was left in a room and the alkali metal elution observed when the solidified body was immersed in water are shown by curves (A) of FIGS. 1 and 2, respectively. As seen from these curves (A), when an inorganic phosphate compound is used as hardening agent, approximately 1% by weight of salt is deposited when the solidified body is left in a room for 400 hours and approximately 8% by weight of alkali metal is eluted when said solidified body is immersed in water for the same period of time.
Thus, the prior art method using an inorganic phosphate compound (P.sub.2 O.sub.5.SiO.sub.2) as the hardening agent of the alkali silicate solidifying filler was attended by the problem that the salts are deposited on the surface of the produced solidified body of radioactive waste because of the formation of easily soluble salt M.sub.3 PO.sub.4 in the hardening reaction, and the deposition of salts causes the deterioration of the water resistance of the solidified body which might lead to the leakage of radioactive nuclides from the solidified body.